In prior art machines for the control of the printed circuits are known.
These machines generally consist of a board or bed bearing a plurality of electrically conductive needles, on which must be disposed the card with the printed circuit and by activating properly the electric conductivity among the different needles and by making the respective measurings it is possible to check whether the printed circuit has defects.
Obviously for such control it is necessary for the needles to contact the printed circuit in the proper position.
As the printed circuits are very different, it results necessary to apply over the needle base board, an adaptation board that comprises the same needles but displaced in a different way in conformity with the printed circuit to be controlled.
The drawback of this solution is the limitation of the machine and the possibility to control only the printed circuits that have at disposal corresponding adapters or adaptation boards.
Needing as many adaptation boards as are the cards with different printed circuits to be controlled.
Furthermore it is known that the layers of the circuits are not always coplanar, wherefore a suitable electric conduction for an effective contact is not always ensured.
EP,A,0 164 722 (Black) Dec. 18, 1985 discloses: an automatic test system for testing printed circuit boards (cards) in which:
In this document prior art is cited:
use of a "bed of nails" (bed or plate or board of needles), in which each nail or needle is fixedly placed in said bed or plate or board. PA1 two needle bearing boards are used, one opposing the other and between them is placed for the analysis the respective printed circuit card to be controlled, said printed circuit card being movably supported on its plane, PA1 characterized in that:
In this solution the "bed of nails" has the nail position absolutely fixed and corresponding to the specific pattern of the printed card to be tested.
Drawback of this solution is the requirement of as many "beds of nails" as the different printed circuit cards to be tested. use of a universal grid matrix having a maximum possibility of positionally fixed needles (nails) and a computer program to activate only the required needles (nails) at a time. The programming being dependant on the pattern of the printed card to be tested.
This solution has the drawback of continuously reprogramming the machine. Furthermore, because the contact nails or needles are bulky, it is impossible to cover all kinds of patterns.
This problem is avoided rendering displaceable along the coordinate axis said contact needles or nails.
Nevertheless this solution requires a very complicated and expensive construction of the testing machine and in this case the needles or nails cannot be placed closely one next to the other. Furthermore a continuous reprogramming is required to move one needle or nail in respect to the other.
Finally, EP,A,0 164 722 discloses its claimed solution, in which there is a magazine for temporarily storing a plurality of probes (nails or needles) and a base receiver member positioned adjacent to the test head (bed or plate) on one side thereof and in a position to contact the base of each probe. The test head (bed or plate) is adapted to receive the probes and position the probes in a pattern matching test points on the printed circuit board. Means are provided for supporting the test head adjacent to the magazine and in a position to receive probes (nails or needles) from the magazine. Means are also provided for transferring selected ones of the probes from the magazine to the test head.
The probes (nails or needles) are for this purpose, movable axially from inside the said test head to the outside or vice versa and some of these further axially moved in order to allow contact with the printed circuit of the card to be tested.
This solution is extremely complex because of the mechanisms to move the probes (nails or needles) and the program to operate. Furthermore this solution does not allow a complete universal testing of all kinds of printed circuit cards because of the limitation of the pattern of the test head.
EP,A,0 468 153 (ATG ELECTRONIC) Jan. 29, 1992, discloses a system in which the respective contact needles are not fixed in position on the respective needle bearing board, but are movable both axially and from their position along the coordinate axis.
In this way it is possible to make the testing machine universal. Nevertheless, this solution is complex and requires a continuous reprogramming. Furthermore this solution is lengthy because the movable needles are obliged to move for a long way to reach the exact coordinates, and this has to be done continuously during the testing of each single card for the respective interested points to be tested in the circuit.
EP-A-0 547 251 discloses a method for testing a micro circuit, where a "wafer holder" and a "test head . . . incorporating a matrix of probes" is provided and wherein said wafer holder is movable on its (X-Y) plane whereas the test head is movable along the orthogonal (Z) direction.
This solution is able to test printed circuit cards because it is conceived for testing on one side only.